1. Field of the Invention
The present invention relates to composite nonwoven fabrics suitable for use as air filtration media, for filtering particulate material from fluid streams.
2. Description of the Related Art
Filter media typically utilized for HVAC air filters that perform at efficiencies less than 99.97% at a 0.3 micron challenge are either glass, cellulose or polymer based. Filters made with media in this performance range are typically referred to as “ASHRAE filters” since the American Society of Heating, Refrigerating and Air-Conditioning Engineers writes standards for the performance of filter media in such applications. Polymer based filter media are typically spunbond or meltblown nonwovens that are often electrostatically enhanced to provide higher filtration efficiency at lower pressure drop when compared to glass or cellulose media manufactured by a wet laid paper-making process.
Electrostatically enhanced air filter media and media manufactured by the wet laid process, more specifically with the use of glass fibers, currently have limitations. Electrostatically treated meltblown filter media, as described in U.S. Pat. Nos. 4,874,659 and 4,178,157, perform well initially, but quickly lose filtration efficiency in use due to dust loading as the media begin to capture particles and the electrostatic charge thus becomes insulated. In addition, as the effective capture of particulates is based on the electrical charge, the performance of such filters is greatly influenced by air humidity, causing charge dissipation.
Filtration media utilizing microglass fibers and blends containing microglass fibers typically contain small diameter glass fibers arranged in either a woven or nonwoven structure, having substantial resistance to chemical attack and relatively small pore size. Such glass fiber media are disclosed in the following U.S. patents: Smith et al., U.S. Pat. No. 2,797,163; Waggoner, U.S. Pat. No. 3,228,825; Raczek, U.S. Pat. No. 3,240,663; Young et al., U.S. Pat. No. 3,249,491; Bodendorf et al., U.S. Pat. No. 3,253,978; Adams, U.S. Pat. No. 3,375,155; and Pews et al., U.S. Pat. No. 3,882,135. Microglass fibers and blends containing microglass fibers are typically relatively brittle, and thus when pleated, break resulting in undesirable yield losses. Broken microglass fibers can also be released into the air by filters containing microglass fibers, creating a potential health hazard if the microglass were to be inhaled.
It would be desirable to provide a means for achieving ASHRAE level air filtration while avoiding the above-listed limitations of known filtration media.